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Cover Page - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2019
Feb. 25, 2020
Jun. 30, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 001-15781    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 04-3510455    
Entity Address, Address Line One 60 State Street    
Entity Address, City or Town Boston    
Entity Address, State or Province MA    
Entity Address, Postal Zip Code 02109    
City Area Code 800    
Local Phone Number 773-5601    
Title of 12(b) Security Common stock, par value $0.01 per share    
Trading Symbol BHLB    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 1.4
Entity Common Stock, Shares Outstanding (in shares)   50,200,155  
Entity Registrant Name BERKSHIRE HILLS BANCORP INC    
Entity Central Index Key 0001108134    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Documents Incorporated by Reference Portions of the Proxy Statement for the 2020 Annual Meeting of Shareholders are incorporated by reference in Part III of this Form 10-K.